Gravure printing is an intaglio process employing one or more engraved gravure printing cylinders. Image areas of each cylinder are engraved by an engraving head of an engraving machine to create cells. The cells vary in volume corresponding to the tonal values in the images.
The quality of the final printed product depends upon engraving the correct cell sizes on the cylinder. The shape and volume of each cell dictates how much ink that cell will hold and correspondingly, how an ink dot will appear in print. Even small variations in cell size can produce changes in dot size noticeable to the human eye. It has been shown in testing that the actual cell volume is more representative of the actual print density than the surface area of the cell. Therefore, it is necessary to calibrate the engraver so that accurate and repeatable cell volumes can be produced.
Past attempts to accurately calibrate an engraver have included the use of precision optical instruments to measure various spatial parameters of the cells in order to estimate actual cell volume. This technique is detailed in Wouch et. al. U.S. Pat. No. 5,293,426 assigned to the assignee of the present application. As detailed in such patent, an optical microscope is used to measure the length and width of the surface of a plurality of test cells. From such measurements, the depth, face area, and volume per unit area of each test cell may be estimated. Using statistical analysis, the average cell width, length, depth, face area, and volume per unit area are calculated. The average cell width, area, or volume per unit area is then compared with a predetermined standard value to compute any variance and to adjust the engraving head accordingly.
Non-contact optical profilers, such as the WYKO Rollscope, a vertical scanning interference microscope, have previously been used to characterize the surface roughness of such products as rubber, paper, ceramics, textured steel and aluminum, adhesives, films, and others. The WYKO Rollscope has also been used to characterize cells formed in anilox rolls used in flexographic reproduction. Each of these applications is characterized by relatively uniform depth, shape, volume and density of surface deformations.